Magnetic focusing system



Aug. 12, 195s 1. R. PIERCE 2,847,607

MAGNETIC FOCUSING SYSTEM Filed April 29, 1955 4 Sheets-Sheet l ELEC TRONFLOW COP/5,5235 45 44 s F/G-5 VZW 5f 63 i# "www /NVENTOR J R PIERCEATTORNEY Aug. 12, 1958 .1. R. PIERCE MAGNETIC FOCUSING SYSTEM 4Sheets-Sheet 2 Filed April 29. 1955 /-NSUFF/C/ENTMAGNET/CFYELD -e--fCORRECT MAGNET/C F/ELD o.: 0.2 a las E (7) BEAM CURRENT /Nl/ENroR By J RP/ERCE ATTORNEY Aug. l2, 1958 Filed April 29, 1953 COLLECTO/J CURRENT J.R. PIERCE 2,847,607

MAGNETIC FocusING SYSTEM 4 Sheets-Sheet 3 MAGNET/C F/ELD /N VEA/TOR J RP/ERCE ATTORNEY Aug, 12, 1958 J, R, PlERcE 2,847,607

MAGNETIC FCUSING SYSTEM Filed April 29, 1953 4 Sheets-Sheet 4 F IG.

A TTORNE Y MAGNnrrc Focusrite sYsrn-M John R. Pierce, Berkeley Heights,N. Il., assigner ts lieti Telephone Laboratories, incorporated, NewYorin N. Y., a corporation of New York Application April 29, 1953,Serial No. 351,993

6 Claims. (Cl. S15- 3.55)

This invention relates to systems for focusing beams of chargedparticles, and more particularly to systems Where an electron beam iscollirnated by magnetic fields for travel over a relatively long path asis characteristic of many forms of cathode ray devices, such as, forexample, traveling wave tubes.

A broad object of the invention is to provide an improved system forfocusing beams of charged particles.

A more specific object of the invention is to effect economies' in themagnetic field necessary for good magnetic focusing of an electron beamin a traveling wave tube to the end that there results a saving in thesize and weight of the equipment used for such focusing.

in a traveling wave tube, an electron stream is made to interact with atraveling electromagnetic wave over a distance a plurality of operatingwavelengths long. "fo this end, an electron stream is projected closelypast an interaction circuit along which the wave is propagat ing. Forefficient operation, it is generally important to keep the electron flowcylindrical to avoid having electrons strike the interaction circuit andto confine the electrons to regions of high signal fields. To minimizetransverse components set up by space charge effects, it is the usualpractice to set up a uniform longitudinal magnetic field along the pathof electron flow. This magnetic field is generally achieved either bythe use of permanent magnets or external solenoids.

l-Iitherto, it has been common practice to employ Brillouin typefocusing with high density electron beams. in focusing of this type, theelectron gun is enclosed in magnetic shield, and the electrons arecaused to spiral as they enter the region of longitudinal magnetic fieldfrom the shielded region. The angular velocity of each electron isproportional to the difference in magnetic fiux encountered in goingfrom the shielded region into the field region. The inward or focusingforce per charge is proportional to the product `of the angular velocityand the longitudinal magnetic field, or effectively the square of themagnetic field. This inward force is adjusted to counterbalance exactlythe sum of outward mutually repulsive forces of the electrons (generallydescribed as the space charge forces) and the outward centrifugal forceof the spiraling electrons. lf, in addition to satisfying this conditionalong the magnetic field region, the electron beam is caused to enterthe magnetic field region initially with zero radial velocity, it willtravel without spreading.

However, both because of the relatively long length of the electron pathand because of the large space charge forces existing in an electronstream of high density, it is found in practice that the solenoids orpermanent magnets necessary to provide a satisfactory uniformlongitudinal magnetic field are often large and bulky, being many timesthe Weight and size of the traveling wave tube alone. For obviousreasons, it is desirable to minimize the size and weight of thisfocusing equipment, and the present invention is directed to this end.

The present invention achieves this end by employing nite States arentICC along the path of flow a succession of regions of longitudinalmagnetic field characterized in that the direction of this fieldreverses in successive regions.

Analysis, of which a more detailed description follows hereinafter, hasrevealed that an essentially non-diverging beam may be obtained if theR. M. S. value of the longitudinal magnetic field in the vicinity of thebeam has the same magnitude as the uniform axial field character isticof Brillouin focusing. t is obvious that for a given average fieldvalue, a larger R. M. S. field value results if the field isconcentrated in a succession of relatively short regions instead ofbeing uniform over a relatively long region. Accordingly, a high R. M.S. Value 0f longitudinal magnetic field in the vicinity of the beam,important for good focusing, can be achieved with a minimum of drivingmagnetomotive force by concentrating the longitudinal magnetic fieldalong a periodic series of short gaps along the beam path. For the casewhere the magnetic flux is provided by a solenoid, periodic focusing inthis way permits an overall decrease in the magnetomotive driving forceof the solenoid necessary. ln my copending application Serial No.351,984, filed `April 29, i953, now Patent 2,84l,739, issued July l,

1958, there are described focusing arrangements of this kind utilizingsolenoids. Where permanent magnets are' utilized to create the desiredmagnetic fiux, a succession of longitudinal magnetic sections provideseven more marked advantages over Brillouin type uniform field focusing.First, a periodic field system of this kind can be made as long asdesired by adding magnets without the fields of the added magnetsreducing the fields of magnets already present. Also, the weight of themagnetic material necessary will be considerably less for the periodicfield system since, unlike the uniform field permanent magnet system forwhich the field must be broad in order to be uniform, no extra fluxneeds to be set up merely to insure uniformity of field along the pathof flow. Moreover, in a succession of regions in which the fieldalternates in direction, the field strength falls off away from the aXismore rapidly than in a succession of regions in which the field is ofconstant direction, and hence such a spatially alternating field storesless total energy because it extends over a smaller volume. Since themagnet weight is governed by the total stored energy, this factorpermits a reduction thereof.

In a preferred embodiment of the invention, a succession of hollowcylinders of a permeable material such as Permalloy or soft iron aredisposed for surrounding spaced portions of the electron path. Then asuccession of small permanent magnets are bridged across successivecylinders for forming a series of converging magnetic enses whichestablish longitudinal magnetic fields of substantially circularsymmetry along portions of the electron path corresponding to the gapsformed between successive cylinders. The pole faces of the permanentmagnets are oriented to introduce a reversal of the longitudinalmagnetic field across portions of the electron path corresponding tosuccessive gaps so that the magnetic field will fall off sharply withdistance away from the electron path.

Various other illustrative embodiments will be described herein, each ofwhich is characterized as establishing along the path of flow asuccession of regions of longitudinal magnetic eld, the direction of themagnetic field reversing with successive regions.

The invention will be better understood from the following more detaileddescription taken in conjunction with the accompanying drawings inwhich:

Fig. l shows in schematic form a longitudinal cross section of anelectron beam system in accordance with the invention;

Fig. 2 shows in longitudinal cross section a helix-type traveling wavetube embodying the electron beam system shown in Fig. 1;

Fig. 3 shows in longitudinal cross section an interaction circuitportion of a traveling wave tube in which a focusing system inaccordance with the invention is incorporated into the interactioncircuit;

Figs. 4 through 6 are longitudinal cross sections of variousmodifications of focusing systems in accordance with the invention; and

Figs. 7A through 7C, and 8 through ll are curves which are useful in anexposition of the principles of the invention.

Referring now more particularly to the drawings, Fig. 1 illustratesschematically an electron beam system lil in accordance with theinvention suitable for incorporation in apparatus utilizing a relativelylong electron beam, such as a traveling wave tube. At opposite ends ofan evacuated elongated envelope lll which, for example, is o'f glass ora suitable non-magnetic metal such as copper, are positioned a source ofa solid beam of electrons 12 and a target 13. The source of electrons 12generally vwill be an electron gun which includes an electron-emissivecathode surface, a heater unit, an intensity control element, and anelectrode system for shaping and ao celerating the electron beam. Thetarget 13 serves as the collector of the electrons at the end of theirpath, and accordingly is maintained at a suitable potential positivewith respect to the cathode of the electron source 112 by means of avoltage source 1d. An electrode member which is kept at a positivepotential with respect to the cathode of the electron gun is disposedalong the path of HOW for providing an accelerating field. In atraveling wave tube, the interaction circuit will generally serve assuch an electrode member. In the electron beam system shown, anonmagnetic conductive coating 19 is applied to the inner surface of theelongated portion of the envelope which is maintained at a positivepotential with respect to the cathode of the electron gun by voltagesource 14% for providing the accelerating eld. For maintaining theelectron flow substantially cylindrical (i. e. having in appreciabletransverse components), a succession of identical tubular cylinders 115of a material having a high permeability, such as, for example, softiron, are disposed coaxial with the path of electron ow around theelongated portion of the tube envelope for serving as flux guides.Successive cylinders are spaced apart leaving gaps 16 of uniform lengthbetween adjacent cylinders. A succession of identical U-shaped permanentmagnets 17 are disposed along the path of electron ilow, for convenienceof illustration alternate magnets being shown disposed on the same sideof the electron path, each suc cessive magnet being bridged across asuccessive gap 16 and having each one of its two pole faces fiush with adifferent one of two adjacent cylinders. Successive magnets are reversedin sense, so that only like poles are common to each of the cylinders.Accordingly, alternate cylinders are flush with like poles, adjacentcylinders are flush with unlike poles, as is shown. Effectively, thesuccession of cylinders serve as a succession of oppositely poled polepieces.

As a result of this arrangement of magnets, there results along the pathof electron flow a succession of regions 18, each corresponding to a gaplo between adjacent cylinders, where there exists a longitudinalmagnetic field of substantially circular symmetry and the di- 'rectionof this longitudinal magnetic field reverses along successive regions.By such a succession of regions of time-constant spatially alternatinglongitudinal magnetic field, the electron beam may be focused.

Each pair of adjacent cylinders and its associated magnet may be viewedas a magnetic circuit which is closed by way of its air gap. Successivemagnetic circuits are arranged to have their air gaps aligned along thepath of electron dow and oriented to have the flux across suctot cessiveair gaps reverse in direction. It is apparent that the relativecircumferential position of successive magnets is unimportant if thereluctance of the cylinders is negligibly low. For purposes ofillustration, it is convenient to position successive magnets 180 apartcircumferentially around the path as is shown in Fig. 1, but, inpractice, to achieve a high degree of circular symmetry it may beadvantageous to dispose two or more magnets circumferentially about eachpair of adjacent cylinders or to make the magnets of toroidal orcylindrical form.

Before examining in more detail specific embodiments incorporatingelectron beam systems in accordance with the invention, it seemsappropriate to analyze with some detail the basic arrangement shown inFig. 1. This analysis is for the case of a solid electron beam but itcan be extended to the case of tubular beams.

lt is assumed that, (a) the magnetic field B is axially symmetric anduniform over the electron path crosssection, i. e.,

(BZ will be written simply as B hereinafter) (b) the electric eld E dueto space charge acts only in a radial'direction, i. e.,

E=Er

The Lagrangian for an electron of radius r in an electric and magneticeld is given by where A is the magnetic vector potential, V the electricwhere n is the charge mass ratio of the electron. From Equation 2 theexpression for r derived from the Lagrangian is,

.. B2 2 oVM T -l--T 17 0 For anr electron at the edge of the beam,

Non

vwhere ro is the beam radius at the entrance of the focusing structureand p is the charge density ofthe electron stream, assumed constant overthe beam cross section structure. Equation 3 now becomes,

.. B2 2 r 2 T p l T M :0

Let us define B0 as the peak value of magnetic field at the axis, and Las the magnet period (equal to twice the distance between magnets). Forconvenience, let us dene assessor For the focusing structure shown inFig. 1 magnetic field at the axis is very nearly given by,

B=B0 cos L Using this value of B,

. 2 2 H-C) (largos 2mg-(29) 1:0 2 w 2 w d where, T=wt and }-l-o(l+cos2T)cr=0 (7) This non-linear differential equation can be solved forvalues of ot and of interest to the study of practical traveling wavetubes.

For convenience it was assumed that the electrons were injected with noradial velocity at the point T=0, i. e., at a point where the magneticfield was a maximum. In practice this condition is realized by adjustingthe position of the electron gun with respect to the first lens or byadjusting electrostatic focusing electrodes near the gun.

Figures 7A through 7C show plots of the beam radius as a function of zfor various values of the parameter a with held constant. For a givenfocusing structure and a given value of beam velocity is proportional tothe square of the peak magnetic field (B02) and is proportional to thetotal D.-C. current in the beam I0, and therefore varying a isequivalent to Varying the peak magnetic eld B0. With a particular valueof magnetic field the perturbations of the beam radius are seen to be aminimum (Fig. 7B) and this is the so-called optimum field. For highervalues of B0 the average radius of the beam is decreased (Fig. 7C) andfor lower values of B0 it is increased (Fig. 7A).

The optimum values of nt (or magnetic fleld) are plotted as a functionof (or beam current) in Fig. 8. For small values of 0.2) the optimumvalue of a is approximately equal to ,8. As is increased the optimumvalue of a gets progressively larger than (it should also be noted thatthe mean radius of the beam is getting progressively smaller). Moreimportant, however, is the fact that the ripples in the beam radius getprogressively larger until for a value of the beam radius diverges tomore than twice its initial radius which for the present can be assumedto be the maximum divergence tolerable irrespective of the value of a.When a and are expressed in terms of familiar tube constants oneobtains,

L 2 twin) B021? Vo where L is twice the magnet spacing, d is the beamdiameter, V0 is the beam accelerating voltage, B0 is the peak magneticeld, and K is the beam perveance. It is evident that given a particulartraveling wave tube, for to be less than 0.6 (and consequently a beamconfined to less than twice the initial diameter) it is convenient toadjust L (the magnet spacing) since this is the only parameter notassociated with the physical constants of the tube.

For small values of of the required value for a is seen to be equal tofor minimum perturbation of the beam radius. This value of a correspondsto a peak magnetic field (B0) equal to V5 times the eld required forBrillouin focusing. However, since this is a varying field, one shouldcompare the R. M. S. value of this eld with the Brillouin eld, in whichcase the two are equal. For higher values of ,8 the mean diameter of thebeam for minimum perturbation was significantly reduced below ro (as isevident from Figure 7B). If the means radius is used in computing therequired Brillouin Held, the two schemes of focusing will again be foundto be equivalent with respect to R. M. S. fields within the accuracy ofthe computer curves. riodic magnetic focusing requires the same totalmagnetic energy in the Vicinity of beam as does Brillouin focusing.However, when the magnetic energy is to be supplied by a permanentmagnet, periodic focusing increases many times the eciency possible, andconsequently permits use of a considerable reduction in the amount ofmagnetic energy necessary to be provided initially.

However, unlike the case of the uniform magnetic field, in the case of aperiodic magnetic eld the focusing cannot be improved by increasing themagnetic eld strength well beyond the theoretical required value.Instead there are encountered regions of magnetic field strength whichcause the beam to diverge and therefore for good focusing the field mustlie within certain well dened regions. A goed insight into the mechanismof this phenomenon can be obtained from the Buttery Diagram (Figure 9)which shows the stable and unstable regions of the Mathieu equation.

A consideration of Equation 7 shows that the-first two terms compriseMathieus differential equation while the last term is due to the spacecharge forces of the electrons. A detailed analysis of this differentialequation is found in a book entitled Theory and Applications of MathieuFunctions by H. W. MacLachlan, Oxford University Press (l9/1-7). if thesolution to the homogeneous equation without space charge (Mathieusequation) divergcs, then it is reasonable to suppose that the additionof the space charge term will not restore stability. (However, theconverse is not necessarily true, i. e., if the lhcnnogeneous part isstable the complete solution is not necessarily stable.) From Equation 7the constants a and q of the standard form of Mathieus equation becomerespectively l and 1/2 and therefore describe a straight line on thestability chart (Figure 9). This line intersects the boundaries of thestable and unstable regions at points which define the values of (thecontant a) which separate the pass and stop bands of periodic focusing.

The curve of Fig. l() illustrates the relationship of collector currentwhich was measured as a function of the magnetic strength of the magnetsfor a fixed magnet spacing and a constant beam current of an arrangementof the kind described in Fig. 1 and illustrates graphically thephenomenon of pass bands and stop bands predicted by the analysis.

A simplied analysis which can be carried out is to assume that along thepath of ow the regions of longitudinal magnetic field are short comparedto the distance separating them, so the succession of focusing fieldsmay be regarded as a series of thin converging lenses. Then if the beamis started in such a manner that it is cylindrical midway between twoadjacent lenses, and if the lenses are chosen of the right strength, theflow will be cylindrical between the next two lenses. rThe convergingeffect Therefore, axially symmetric pe of the lenses is on the averagejust balanced out by the diverging effect of the space charge betweenthe lenses, and the electron beam iiow is identical between eachsuccessive pair of lenses. By inserting the quantitative expressions forthe diverging effect of the space charge kgiven in section 9.2 (pages147 through 152) of my book entitled Theory and Design of ElectronBeams, published by D. Van Nostrand Company, Incorporated, New York(1949), it can be shown that for non-divergent fiow it is important that[1/2 z 174W1 E 2-16 Where z is the spacing of successive lenses, ro isthe beam radius at the lenses, l is the current in amperes of the beam,and V is the accelerating voltage acting on the beam. Additionally, itcan be shown that for the results stated the convergence C of the lensrequired is such that where Z is the parameter measuring distance givenby I 11/2 Z 174VST4 6 and the R9 corresponding to a given Z can be foundfrom the graph of Fig. .ll where R is plotted as a function of Z. Hereit can be seen that no values of R0 exist for which Z is greater than2.16. For values of Z smaller than this, there are two possible valuesof R0', one for which -Ro is less than .92 corresponding to a very weaklens and a large minimum beam radius, the other for which -Ro is greaterthan .92 corresponding to a strong lens and a small minimum beam radius.

Fig. 2 illustrates how a typical helix-type traveling wave tube, forexample, of the kind described in United States Patent 2,575,383 whichissued on November 20, 1951 to L. M. Field can be adapted for use withan electron beam system in accordance with the invention. ln theinterest of simplicity, the traveling wave tube is shown in schematicform, many of the specific tube details being omitted. However,reference can be had to the abo-veidentied Field patent for a moredetailed description of a typical helix-type traveling wave tube. Thevarious tube elements are enclosed in a non-magnetic envelope 20.Alternatively, it is possible to utilize an envelope of a magneticmaterial such as lrovar so long as it is made sufficiently thin as tobecome magnetically saturated without seriously reducing the magneticfield. At one end to serve as a source of electrons there is positionedan electron gun which comprises an electron emissive cathode Z2, and anelectrode system for shaping and focusing the electrons emitted into abeam including a beam forming electrode Z3 and an accelerating anode 24.At the opposite end of the envelope, a collector 2.5 is positioned intarget relation with the electron gun. Disposed along the path ofelectron flow is a helically coiled conductor 26, a plurality ofoperating wavelengths long, which serves as the interaction circuit forpropagating a slow electromagnetic wave in coupling relation with theelectro-n beam and as an electrode for accelerating the electron beam.

The helix 26 is joined at opposite ends to an input coupling strip 2&7by an impedance matching section 2d and to an output coupling strip 29by an impedance matching section 30. rl`hese matching sections 23 andEll are simply extensions of the conductor 26 in which the pitch of thehelix is gradually increased. An input wave is applied to the upstreamend of the interaction circuit by way of input wave guide couplingconnection 31 and the output wave is abstracted at the downstream end byway of output Wave guide coupling connection .32.. Each of the waveguide coupling connections 31 and 32 is a section of rectangular waveguide which has a pair of opposite side walls apertured for passagetherethrough of the tube envelope, and which has a closed end and anopen end by which it can be connected into a wave guide transmissionsystem. Each of the input and output coupling strips 27 and 29 issupported in its corresponding wave guide coupling connection. Inputwaves are applied to the input wave guide coupling connection 31 to havea mode of propagation having an electric iield vector parallel to thecoupling strip 27. In this way, an electromagnetic wave is introducedinto the interaction circuit for travel therealong in a couplingrelationship with the electron beam. The electron gun forms a solidcylindrical electron beam for projection coaxially through the helix.For accelerating the electron beam longitudinally, the helical conductoris maintained by `suitable lead-in connections (not shown) at apotential which is 'tive with respect to that of the cathode 22 andwhich ty approximately the same as that of the collector orsubstantially lower. For eiiicient operation, it is important that theelectron flow be substantially parallel to the axis of the helix 26whereby a minimum of electrons is lost in striking the conductor,Accordingly, it is desirable to introduce some focusing of the electronbeam in order to counteract the radial space charge forces which tend tomake the beam diverge.

in accordance with an embodiment of the invention, each of a successionof cylinders 3E of a material having a high permeability, such as softiron, Permalloy, or one of the ferrites, is disposed around the tubeenvelope spaced apart along the axis in the region of electron dow forforming a succession of gaps 39 between adjacent cylinders. In theregion of the input and output wave guide coupling connections 3l and32, special precautions must be taken. To minimize the discontinuitiesto be introduced at these regions, it is desirable that the two sidewalls of each of the wave guides 31 and 32 which are apertured for thepassage of the tube envelope also be of material of high permeabilitywhile the other two `side walls and the end closures 35 of each be of anon-magnetic metal such as copper. In ythis way, each of the aperturedside walls serves as a separate pole piece and the space between thesewalls serves as another gap.

Across each of the gaps 39 there is bridged a permanent magnet 36 which,for example, can be of the horseshoe type, having its pole faces liushwith an adjacent pair of cylinders. Again, it is convenient for purposesof assembly and illustration to dispose successive magnets on oppositesides of the envelope. In accordance with a characteristic feature ofthe invention, successive magnets are oriented in opposite senseswhereby the direction of the longitudinal magnetic eld across successiveregions of the electron path corresponding to gaps 39 between cylindersis reversed. Permanent magnets 37 are similarly bridged across the twoapertured side walls of each of the two coupling connections 31 and 32in accordance with the practice of treating each of these apertured sidewalls as pole pieces. It may be that, because the spacing between theseside walls is, because of transmission considerations, preferablyditferent from the optimum spacing between pairs of regular cylinders,the size of each of permanent magnets 36 may be proportionatelydifferent from that of permanent magnets 37. Alternatively, it may bedesirable to taper the wave guides 31 and 32 `to provide a wallseparation in the direction of ow more nearly equal that desired for thegap separation 39 between adjacent cylinders 38.

The various parameters, such as gap spacing and magnetic intensities, ofan electron beam system of this kind are adjusted in accordance with theprinciples set forth in connection with the detailed analysis of thearrangement shown in Fig. 1.

Moreover, an electron beam system of this kind can similarly beincorporated into various other forms of traveling wave tubes. Inparticular, in some forms, elements of the focusing structure can beincorporated as part of the interaction circuit. For example, Fig. 3shows a fragment of traveling wave tube utilizing an narradorinteraction circuit of this kind. A regular succession of annular rings,adjacent rings all and d2 being alternately of a magnetic metal such assoft iron and a non-magnetic metal such as copper, are stacked .togetherto form a cylindrical wave guide structure through which is projected anelectron stream. Each ring has substantially the same outer diameter,but alternate iron rings All have a smaller inner diameter whereby suchrings project further into the hollow of the cylindrical wave guide andare closer to the electron flow. Additionally, it may be advantageous totaper outwardly the innermost ends of such rings for narrowing the airgaps i therebetween adjacent the path of tiow. rthere consequentlyresults a corrugated wave guide which is a well known form of slow waveinteraction circuit suitable for incorporation into traveling wavetubes. ln accordance with the principles of the invention, a successionof permanent magnets d3 is disposed along the interaction circuit, eachmagnet having its pole faces flush with a pair of magnetic rings,successive magnets being reversed in sense. ln this way there is createdalong the path of electron flow a succession of regions of longitudinalmagnetic field corresponding to the air gaps dii, the direction of themagnetic field reversing along successive regions. Various other slowwave circuits can be deviced of this kind in which portions are ofsuitable magnetic material for serving as flux guides in magneticfocusing systems operating in accordance with the spirit of theinvention.

Various other magnet arrangements are possible for instrumenting theprinciples of the invention. By way of example, several otherembodiments of electron beam systems are shown in Figs. 4, 5 and 6.

In the arrangement of Fig. 4, a succession of annular cylinders lll ofmaterial having a high permeability are disposed along the path ofelectron flow for serving as pole pieces and spaced apart for forming asuccession of gaps 46. A series of bar magnets t7 is disposed across thesuccessive gaps, the magnets across adjacent gaps being reversed insense whereby there results along the path of electron tlow a successionof regions of longitudinal magnetic field corresponding to successivegaps, the direction of the longitudinal magnetic t'ield reversing withsuccessive sections. To increase the circular symmetry it may bedesirable to substitute for the bar magnets i7 annular cylindricalmagnets magnetized in an axial direction.

In the arrangement of liig. 5, a succession of annular cylindricalmagnets 5l magnetized in an axial direction are spaced apart along thepath of electron flow, adjacent magnets being reversed in polarity.Here, again there results along the path of electron liow a successionof regions of longitudinal magnetic held, the direction of thelongitudinal field reversing with suc ive regions.

Fig. 6 shows still another possible arrangement. A succession of annularsections 61 of material suitable for being permanently magnetized, suchas Alnico, is disposed in contiguous relationship along the path ofllow'. Each section is a tubular circular cylinder whose inner surfaceis grooved to form an annular air gap 62 surrounding and having anopening 63 along a relatively smaller region into the electron path.There consequently results a regular series of openings 63 along theelectron path. A few turns of wire 64 suitable for carrying largecurrents are wound in each of the gaps 62. Then, after the varioussections are firmly positioned in place, a direct current is passedthrough the wires 64 for permanently magnetizing the various sections tothe desired intensity. The direction of the current in the turns in thevarious gaps is adjusted so that successive sections are magnetized inopposite senses as illustrated by the polarities shown in Fig, 6. As aconsequence, there results along the electron path a succession ofregions of longitudinal magnetic iields, corresponding to successiveopenings 63, the direction of the magnetic field reversing with eachsuccessive region. ln arrangements 10 i i of this kind, the optimummagnetic field intensity for a given spacing for the successive magnetsand a given beam current can be arrived at conveniently by adjusting theamplitude of the magnetizing current through wires 64 for maximumcollector current. By magnetizing in this way after assembly there isavoided the possibility that the intensity of the eld of the magnetswill be changed away from the desired value in the assembly process as aresult of forcing like poles of adjacent magnets into close proximity.Alternatively, however, it is possible to assemble in the first instancea plurality of individual permanent magnets of the coniiguration shownand so to avoid the need for subsequent magnetizing.

It can be seen from the foregoing description that the variousembodiments described are merely illustrative of the principles of thegeneral invention. Various other arrangements can be devised by a workerskilled in the art without departing from the spirit and scope of theinvention. For example, the desired succession of longitudinal magnetictield regions' with the direction alternating with successive regionscan be derived by a quadrupole magnetic structure of the kind describedin copending application Serial No.` 351,977 led April 29, 1953, by P.l. Ciofli, now Patent 2,844,754, issued July 22, 1958. Additionally, asdescribed in the Cioth yapplication, it may be advantageous to provideflux guiding means for aligning the magnetic flux in successive gaps,

Moreover, in a copending application Serial No. 351,874, filed April 29,1953 by J. T. Mendel there are described arrangements for betteradapting the principles of the present invention to the focusing ofhollow electron beams. Alternatively, arrangements can be devised inwhich the two poles of each of a series of permanent magnets areassociated with other than adjacent pairs of pole pieces.

What is claimed is:

l, In combination an electron source and target electrode definingtherebetween a path of electron flow, a helical conductor disposed alongthe path of flow for accelerating the electron beam and for propagatingelectromagnetic waves for interaction with the stream, a succession ofcylinders of high permeability material disposed around and spaced apartalong the path of flow, input and output wave guide sections disposedalong the path of ow in coupling relation to the input and output endsof the helical conductor, a pair of opposite side walls of each waveguide section being apertured for passage therethrough of the electronliow and being of a high permeability material, and a succession ofpermanent magnets disposed along the path of flow for bridging acrosssuccessive cylinders and said apertured side walls of said wave guidesections for forming along the path of ow a succession of regions oflongitudinal magnetic field, the direction of the eld reversing withsuccessive regions.

2. A traveling wave tube comprising an evacuated envelope, means forforming a cylindrical beam of electrons for tiow axially through saidenvelope, an interaction circuit for propagating a slow electromagneticwave in field coupling relation with said beam, and means formaintaining the electron beam cylindrical and of substantially uniformdiameter during its progression past said interaction circuit, saidmeans comprising a succession of identical pole pieces spaced equaldistances apart along the path of said liow, and a plurality ofsubstantially identical magnet means comprising a plurality ofsubstantially U-shaped permanent magnets having poles abutting adjacentof said pole piece cylinders interposed between adjacent pole pieces,each of said pole pieces being common to like poles of two adjacentmagnet means and each adjacent pair of said pole pieces dening a gap ofthe same length as the other of said gaps, whereby said pole pieces andmagnet means provide a longitudinal region of periodic spatiallyalternating magnetic Held along the axis of the electron beam.

3. A traveling wave tube comprising an evacuated envelope, means forforming a cylindrical beam of electrons for flow axially through saidenvelope, an interaction circuit for propagating a slow electromagneticwave in field coupling relation with said beam, and means formaintaining the electron beam cylindrical and of substantially uniformdiameter during its progression past said interaction circuit, saidmeans comprising a succession of identical pole pieces spaced equaldistances apart along the path of said ow, said pole pieces forming saidinteraction circuit, and a plurality of substantially identical magnetmeans interposed between adjacent pole pieces, each of said pole piecesbeing common to like poles of two adjacent magnet means and eachadjacent pair of said pole pieces dening a gap of the same length as theother of said gaps, whereby said pole pieces and magnet means provide alongitudinal region of periodic spatially alternating lield along theaxis of the electron beam.

4. A traveling wave tube comprising an evacuated envelope, means forforming a cylindrical beam of electrons for flow axially through saidenvelope, an interaction circuit for propagating a slow electromagneticwave in tield coupling relation with said beam, and means formaintaining the electron beam cylindrical and of substantially uniformdiameter during its progression past said interaction circuit, saidmeans comprising a succession of identical pole pieces spaced equaldistances apart along the path of said liow, said pole pieces comprisingannular cylinders having extending nose portions axial of said envelope,and a plurality of substantially identical magnet means interposedbetween adjacent pole pieces, said magnet means comprising permanentmagnets positioned axially of said envelope and extending betweenadjacent of said annular cylinders, each of said pole pieces beingcommon to like poles of two adjacent magnet means and each adjacent pairof said pole pieces defining a gap of the same length as the other ofsaid gaps, whereby said pole pieces and magnet means provided alongitudinal region of periodic spatially alternating magnetic fieldalong the axis of the electron beam.

5. In a traveling wave tube, an evacuated envelope, means for forming auniform cylindrical electron beam for flow axially through saidenvelope, an interaction circuit extending within said envelope parallelto the electron beam for propagating a slow electromagnetic wave incoupling relation with the electron beam, and means for overcoming spacecharge forces in said beam and maintaining the electron flow cylindricaland of substantially 'constant diameter in its passage past saidinteraction circuit, said means comprising a plurality of identicaltubular cylinders of material having a high permeability spaceduniformly apart and coaxially with the path of electron ilow, and aplurality of identical permanent magnets, each extending between twosuccessive cylinders, said cylinders constituting pole pieces and eachpole piece serving as a common bridging point for like poles of adjacentpermanent magnets, successive magnets thereby being reversed in sense,and alternate cylinders being of the same magnetic polarity, thesuccession of cylinders serving as n of oppositely poled pole piecesforming a longitudinal region of periodic spatially alternating magneticeld sinusoidal in effect along the axis ofthe electron beam.

6. ln a traveling wave tube, an evacuated envelope, means forming acyiindrical beam for flow axially through said envelope, a conductivemember extending within said envelope parallel to the electron beam forpropagatinga slow electromagnetic wave in coupling relation with theelectron beam and establishing an electrostatic eld for accelerating theelectron beam, and means for overcoming space charge forces in said beamand maintaining the electron flow cylindrical in its passage past saidconductive member comprising at least four pole pieces spaced apartalong the beam path and at least three permanent magnets disposed alongthe path of iiow external to the envelope, each magnet extending betweena pair of adjacent pole pieces, successive magnets being reversed inpolarity whereby adjacent pole pieces are oppositely poled.

References Cited in the tile of this patent UNITED STATES PATENTS2,200,039 Nicoll May 7, 1940 2,218,725 Schroeder Oct. 22, 1940 2,233,264Marton Feb. 25, 1941 2,259,994 Boersch et al Oct. 21,1941 2,296,355Levin Sept. 22, 1942 2,300,052 Lindenblad Oct. 27, 1942 2,305,884 LittonDec. 22, 1942 2,369,796 .Ramberg Feb. 20, 1945 2,418,349 Hillier et al.Apr. 1, 1947 2,503,173 Reisner Apr. 4, 1950 2,640,162 Espenschied et alMay 26, 1953 2,741,718 Wang Apr. 10, 1956 OTHER REFERENCES Article by E.D. Courant et al., pages 1190-1196, Phys. Rev. for December 1952.

